CALCIUM SELENITE

Description

Calcium selenite has the molecular formula of CaSeO3 and the molecular weight of 167.0410 g/mol. It can be prepared by the addition of sodium selenite to a solution of magnesium chloride:
CaCl2(aq)+ Na2SeO3(aq) CaSeO3·H2O(solid)
It can also be prepared by treating a solution of calcium chloride with selenous acid and adding sodium carbonate to start precipitation. The monohydrate is a colorless crystal with a density of 2.09 g/cm7.27 that is insoluble in water. The Ksp is 1.2×10^-7.27. It is soluble in dilute acids. A dihydrate has also been observed whose molecular weight is 203.0669 g/mol. The monohydrate has a molecular weight of 185.0569 g/mol and is a monoclinic crystal with a=7.622, b =6.745 , c=7.922, b=108.46, V=386.4 ³, Z=4 with a density of 3.18 g/cm3. The polyhedron around the calcium atoms is pentagonal bipyramidal and the selenite group forms a trigonal pyramid with Se and O atoms at the apices. These pyramids form layers parallel to the bc plane which are connected by hydrogen bonds involving crystal water. The selenite group acts as a bidentate ligand.
The chemistry of the system CaO–SeO2–H2O has been investigated at 23 and 80°C. The existence of four major phases in this system:
CaSeO3·H2O
Ca(HSeO3)·2H2O
Ca2(SeO3)(Se2O5)
Ca2(HSeO3)2
These have been confirmed, and the equilibria among them determined. At 23°C, CaSeO3·H2O and Ca(HSeO3)·2H2O are the stable calcium selenites. CaSe- O3·H2O exists over the pH range 12.5–3.7, making it the most important phase for natural systems as well as most technological applications. Ca(HSeO3)·2H2O is the more acidic phase; it exists over the pH range 3.7– 0.4. Incongruent dissolution of Ca(HSeO3)·2H2O forms CaSeO3·H2O. These results indicate Ca2(HSeO3)2(Se2O5) is not a stable phase at 23°C. At 80°C, CaSeO3·H2O, Ca2(SeO3)(Se2O5), and Ca2(HSeO3)2(Se2O5) are the stable calcium selenite phases. Ca(HSeO3)·2H2O has not been observed. Thus, CaSeO3·H2O exists over the broadest pH range, 11.0–3.9. Ca2(SeO3)(Se2O5) and Ca2(HSeO3)2(Se2O5) dissolve incongruently.

Uses

CALCIUM SELENITE is used as a feed additive to replace sodium selenite, and its function in feed is the same as that of sodium selenite, but with reduced toxicity.